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Re: Unresponsive OB-Xa - pot board issues?

Posted: Thu May 02, 2013 2:31 am
by DesolationBlvd
With the pot boards disconnected, IC 5 of the pot board measures +0.771V and -0.826V at 7 and 16, respectively.

Re: Unresponsive OB-Xa - pot board issues?

Posted: Sun May 05, 2013 8:12 am
by radek tymecki
the first rectifier is always getting too hot. that caused oberheims to freeze for some time.

i had a problem once with pot boards. i can remember i've replaced 4051 on pot board and it started working again

Re: Unresponsive OB-Xa - pot board issues?

Posted: Sun May 05, 2013 6:31 pm
by DesolationBlvd
radek tymecki wrote:i had a problem once with pot boards. i can remember i've replaced 4051 on pot board and it started working again
I noticed that there are three 4051s: ICs number 1, 5, and 6.

Which way do they go in? (as in, which way will the writing on the chip be facing when the lid is flipped up - read top-to-bottom or bottom-to-top?) I want to rule out a chip being in backwards, and also want to make sure I am putting the replacements in correctly.

Re: Unresponsive OB-Xa - pot board issues?

Posted: Sun May 05, 2013 7:19 pm
by radek tymecki
check in service manual. 4051 is pretty cheap so exchange all 3 :)

Re: OB-Xa pot board issues?

Posted: Fri May 10, 2013 4:10 am
by DesolationBlvd
The new 4051s came in. They didn't work. I put the old ones back in...the OB-Xa sprang back to life! Apparently some were in backwards! :oops: (where's a facepalm smiley when you need it?)

Now I'm back to where I started, roughly - tuning and control issues, and I've narrowed down the causes.

Many of the front panel issues could be the result of worn-out connector cables between the pot boards - fiddling with them led to wild changes.

Also, I found that the bender offset is wrong, and I have an early mod box that does not have the trim pot for that.

Lastly, I think I need more lube in the keybed. The keys are still very stiff.

Re: OB-Xa pot board / bender issues?

Posted: Fri Jul 12, 2013 4:37 am
by DesolationBlvd
Forgot to bump. I'm still ready to fix this.

Re: OB-Xa pot board / bender issues?

Posted: Tue Jun 10, 2014 6:18 pm
by DesolationBlvd
Bump. My tech said the EG chips were shorting out on the -5V rail when he tested them individually...So is this a basket case?

Re: OB-Xa - all EGs gone bad?

Posted: Tue Jun 10, 2014 8:36 pm
by Jacob Hunt
I had alot of similar problems with the connecting ribbon cables myself. Sometimes the envelopes would be completely different between boards. I also one time had a slight difference between them that was fixed by calibrating the power supply. The pot board 4051's have been pretty problematic too. The tl084 chips on the bottom board (I believe it was the lower right hand side bunch kept the autotune from working properly). I have also had bad tuning problems caused the bender module, so disconnected it for now, and that helps stability alot if the old pot is bad/dirty.

Re: OB-Xa - all EGs gone bad?

Posted: Wed Jun 11, 2014 4:16 am
by synthparts
I have the EG chips in stock (CEM3310) if you want to fix it. Are all of them really bad?

Re: OB-Xa - all EGs gone bad?

Posted: Wed Jun 11, 2014 6:20 am
by DesolationBlvd
Yeah, check my original post because I've updated it.

Actually, the tech name-dropped you. I have an idea - maybe I have the tech get only four out of the 16 CEM3310s. That way, we can test the upper and lower trays - two voices, to see if it's really bad chips or more power supply issues.